Enzyme based biosensors are highly selective devices which rely on the specific binding of the target analyte (the substrate) to the active-site regions of the enzyme. The enzymatic reaction between mushroom tissue polyphenol oxidase and its substrate phenol is coupled with the use of potassium ferrocyanide (K4Fe(CN)(6)) as the mediator in ammonia buffer solution (pH 8.80). The response of devices is often affected by the presence of inhibitors, which combine with the free enzyme in a manner that prevents substrate binding. The objective of this study is to describe an efficient and yet simple strategy to eliminate the interference of metals in amperometric biosensing. The methods relies on in situ removal of the interfering metal by a surface-bound complexing agent, disodium ethylenediaminetetraacetate (EDTA). EDTA is incorporated into the electrode by mixing it within the enzyme-containing carbon paste matrix. Mixed enzyme/carbon paste electrodes are receiving considerable attention for the preparation of fast responding biosensors.